I. Field of the Invention
This invention relates to compositions and processes for thermoplastic processing of PET, more particularly extrusion/melt shaping of PET.
II. Description of the Prior Art
Until now, linear thermoplastic polyesters such as PET have found no utility in extrusion/melt shaping and related thermoplastic fabrication techniques that require dimensional stability in the melt because such techniques require high melt viscosity and a high degree of melt strength and elasticity. PET generally has an intrinsic viscosity of about 0.5 to 1.1 dl. per gm. and insufficient melt strength and elasticity for such applications. Furthermore, PET exhibits a fast rate of crystallization at temperatures above 140.degree. C. which makes the achievement of clear amorphous articles by such thermoplastic fabrication techniques difficult. Therefore, until now, articles produced from PET had to be made by injection blow-molding techniques in which a parison or perform is injection molded, cooled rapidly and then reheated to a temperature above the T.sub.g but below the crystalline melting point and then blown to the desired shape. See U.S. Pat. Nos. 3,733,309; 3,745,150; and 3,803,275. While amorphous articles would be preferred because of their clarity and toughness as compared to crystalline articles, until now such processing required very specialized equipment such as is shown in U.S. Pat. No. 3,803,275 wherein a hollow slug was extruded directly into a mold maintained at less than 0.degree. C.
It has been previously suggested by Dijkstra et al, U.S. Pat. No. 3,553,157, to prepare thick-walled shaped articles of improved impact strength from PET and a compound capable of reacting with hydroxyl or carboxyl end groups, for example polyanhydrides. "Thick-walled" is defined by Dijkstra et al as "shape and/or dimensions are such that they are not readily conducive to orientation of the polymer by drawing." Dijkstra et al prefer crystalline articles reinforced by glass fibers, and teach nothing with regard to methods of producing blow-molded articles, blown film or foam from PET, nor anything regarding enhancement of melt characteristics of PET.
Extrusion/melt shaping of poly(butylene terephthalate) (PBT) at intrinsic viscosities at least 1.05 dl./gm. has been accomplished by a variety of techniques. See U.S. Pat. Nos. 3,814,786 and 3,931,114. Borman et al, Ser. No. 382,512 of July 25, 1973 (Netherlands 74,07268) attempt to solve this melt strength problem by the use of branched polyesters. The branching necessarily must be conducted in the polyester kettle and thus there is an upper limit as to how much viscosity Borman et al can achieve while still being able to handle the branched polyester.
The object of the present invention is to provide a method of thermoplastic processing of PET to form amorphous articles. It is a further object to provide amorphous extrusion/melt shaped PET articles. A still further object is to provide clear PET bottles by extrusion blow-molding.